One method of getting more light out of a lamp is to increase the size of the filament and then increase the filament current. The current increase brings the larger filament back up to the proper temperature, so the enlarged filament emits more total light. Unfortunately, the current increase must be born by the existing lead structure. Any points of electrical resistance in the socket, seal and other portions of the lead structure then get hotter. The extra heat in the socket or seal can injure the socket, or shorten the lamp's life. There is then a need to improve the electrical lead structure of press sealed lamps.
In small pin type lamps, the filament is commonly press sealed in a small quartz tube. The filament leads join to molybdenum seal foils that are in turn welded to round nickel or molybdenum external leads. The round external leads are capped with nickel tubes that are then swaged to the external leads near where the leads emerge from the quartz. The rest of the nickel capping tube is then flattened, crushing the round tube and the enclosed round wire into an approximately rectangular blade connector. Typically both external leads are capped, and both nickel tubes are flattened to extend as offset, parallel but not coplanar blades. The side by side blades form a plug connection that may be inserted into a lamp socket.
The flattened tube construction starts to electrically fail at about 350 or 400 watts. The contact area between the external leads and the flattened tubes is too small, and irregular. Local hot spots may form along the external leads. Similarly, the seal foil to external lead weld may have too small a conduction area. The blades can overheat, and the seals can fail. There is then a need for a blade type lead structure for small lamps that has improved conductivity, and one where the improved conductivity is sufficient to withstand 400 watt service.